That would be Lore-master or Captain, and to a lesser extent Burglar (until HD).
Lore-master is less noticed but has a greater impact on tough fights.
Captain focuses more on buffing allies, Lore-master more on debuffing enemies, so in most group content (= easy content) a Captain is more welcome because a Captain speeds things up. In that same easy content a Lore-master will often just go pure DPS because debuffing isn't required. Difficult content is where a Lore-master shines.
Burglar has debuffs too, but they are currently not as effective and are mostly used to raise group DPS. They have high single-target DPS and are therefor often used as DPS class rather than support.
[I]In the sea without lees standeth the Bird of Hermes.
[/I][I]When all his feathers be from him gone, He standeth still here as a stone.
Here is now both white and red, And all so the stone to quicken the dead[/I][I].
The Bird of Hermes is my name, Eating my wings to make me tame.[/I]